Method of fractionating mixtures of particulate solid materials



Nov. 7, 1967 w. RESNICK ETAL 3,351,202

METHOD OF FRACTIONATING MIXTURES OF PARTICULATE SOLID MATERIALS OriginalFiled Dec. 16, 1963 INVENTORS- WILL/AM mam/ MOEQECI/A/ Are/[L UnitedStates Patent Ofiice 3,351,202 Patented Nov. 7, 1967 3,351,202 METHOD OFFRACTIONATING MIXTURES F PARTICULATE SOLID MATERIALS William Resnick,Haifa, and Mordechai Avriel, Kiryat Yam, Israel, assignors to TechnionResearch and Development Foundation, Ltd., Haifa, Israel, a corporationof Israel Continuation of application Ser. No. 331,345, Dec. 16,

1963. This application Oct. 17, 1966, Ser. No. 587,346

8 Claims. (Cl. 209-474) ABSTRACT OF THE DISCLOSURE The inventionconsists in a method of fractionating mixtures of particulate solidsinto fractions of, respectively, finer and coarser particles, and/orspecifically lighter and heavier particles. A gas is passed upwardlythrough a plurality of zones, the coarser particles dropping down, andthe finer particles rising and being removed.

This application is a continuation of copending application Ser. No.331,345, filed Dec. 16, 1963, now abandoned.

The invention is based on the observation that in a fluidized bed finerand/or specifically lighter particles tend to rise to the top andultimately to be discharged as overflow or overhead, while coarserand/or specifically heavier particles sink to the bottom of the bed andare ultimately discharged as the underflow or bottoms In the separatingmethod according to the invention, the mixture of solids to befractionated is fed, continuously or in batches, into a median zone offluidized bed constituted by particles of the same kind as those of themixture and a fraction of finer and/ or specifically lighter particlesis withdrawn, continuously or in batches, from the top of the bed, and afraction of coarser and/or specifically heavier particles from thebottom of the bed.

In a preferred embodiment of the invention, fractionation is effected bymeans of a series of fluidized beds preferably superposed upon oneanother, the feed being introduced into a bed about midway in theseries.

The series of fluidized beds may consist either of a plurality ofseparate beds or of a column subdivided by bafiles into a series ofsuperposed chambers intercommunicating for admitting the downward flowof the coarser and/ or specifically heavier particles and the upwardflow of the gaseous fluidizing medium and the finer and/or specificallylighter particles.

In a column serving for the purposes of the present invention, thedesign of the passages leading from chamber to chamber past the bafllesis immaterial. For example, the baffles may extend to the wall of thecolumn over the greater part of their circumference except for a segmentwhich is left open; or the baflies may be discs substantially similar inshape to the cross-section of the column but smaller than the latter,whereby a gap is left between the circumference of each baflle and thewall of the column; or the baflles may extend across the entirecross-section of the column and have one or more openings.

In some cases, depending on the nature of the material to befractionated, it may be advantageous to provide unequal verticaldistances between consecutive baffles, preferably so that the distancebetween each two baffles increases in the upward direction.

In the performance of the method according to the invention it issometimes preferred to form a fixed bed of the coarser or specificallyheavier particles at the bottom of the column, through which bed theincoming gaseous fluidizing medium passes on its way upwards. This fixedbed is a kind of filter retaining finer and/or specifically lighterparticles so that the output of the underflow at the bottom of thecolumn comprises as far as possible the desired range of coarser and/orspecifically heavier fraction only.

The fractions thus obtained by a first fractionating operation may besubjected to further fractionation in a similar manner, if it is desiredto separate the original mixture into a larger number of sharply definedfractions.

A fluidized-bed column for use in the method according to the inventionis illustrated, by way of example only, in the accompanying drawings inwhich:

FIG. 1 is an axial section of the column;

FIG. 2 is a horizontal section thereof on line IIII of FIG. 1.

The column here illustrated comprises an upright cylindrical shell 1having a conical bottom 2, and a lid 3. Between the bottom 2 and thecolumn 1 a perforated plate 4 is interposed. At its lowest point theconical bottom 2 has an inlet 5 for the gaseous fluidizing medium, e.g.air, and at a somewhat higher level a discharge duct 6 for the coarserand/ or specifically heavier fraction. The lid 3 is provided with anoutlet 7 for the spent gaseous fluidizing medium, while a short distancebeneath the lid a lateral discharge duct 8 for the overflow is proviedin the wall of the column 1.

In the example here illustrated the column contains five bafiles 9,formed each by a circular disc (see FIG. 2) of which a segment is cutoff so as to leave a port 10 for the upward passage of fluidizing mediumand finer and/0r specifically lighter fraction, and a downwards passageof the heavier fraction. The parts 10 are angularly staggered by frombaflle to bathe, as shown in FIG. 1. Both the ascending solids and gasand the descending solids are thus compelled to travel along a tortuouspath.

About in the middle of the height of the column 1 between the baflles 9aand 9b, a feed inlet 11 is connected to the column.

. The invention is illustrated by the following examples to which it isnot limited.

Example 1 The column was designed as described above with reference tothe drawings. The column was made of a transparent polymethylmethacrylate (Perspex, trademark). It had an inner diameter of 63 mm.and a height of 1.30 m., measured from plate 4 to the lid 3. Itcontained five baffles whose mutual distances grew from baflle to bafflein the upward direction.

The feed was a sand mixture of substantially equal parts by weight of acoarse fraction of a particle size of 30740 mesh, and a fine fraction ofa particle size of 70-100 mesh. An attempt was made to separate thesetwo fractions from each other as completely as possible in oneoperation, with air as fluidizing gas. The following Table I shows theextent to which the attempt succeeded- It is seen that the recovery ofcoarse particles in the underflow, i.e. the uniformity of the fractionas regards particle size, decreases as the rate of flow of thefluidizing Example 2 The feed was a mixture of substantially equal partsby weight of sand having a bulk density of 2.65 g./cc. and an averageparticle size of 20-30 mesh, and an organic material having a bulkdensity of 1.44 g./ cc. and the same average particle size as the sand.

Fractionation according to specific gravity gave the results tabulatedin Table II below:

TABLE 11 Rate of air- Rate of feed Underflow Overflow flow (liter/(gJseeond) (percent of (percent of orsecond) sand) genie matter) Thistable shows the curious result that while the underflow was practicallypure sand irrespective of the rate of flow of the fiuidizing air, theproportion of organic matter in the overflow dropped as the rate of flowof fiuidizing gas increases. This phenomenon was probably due to somepart of the organic matter being carried off with the fiuidizing gasthrough the gas outlet 7, which could not be recovered at the dischargeduct 8.

What is claimed is:

1. A method of fractionating mixtures of particulate solids havingcoarse and fine particles, which comprises providing means forming aplurality of at least three superposed zones in communication with eachother and separated by a plurality of baffles therebetween defining anupward tortuous path from one side of a zone to another side of anadjacent zone, the space between said baflles increasing in an upwarddirection and said baffles having openings of limited size only at thesides thereof, introducing a mixture of particulate solids into one ofsaid zones, introducing an inert gas at ambient temperatures into thelowermost zone at a suflicient velocity to fluidize fine particles ofsaid mixture, said velocity being sufliciently low to cause the coarsestparticles to fall down into the lowermost zone and to form a bedtherein, periodically discharging said coarsest particles from thebottom of said lowermost zone, causing a mixture containing said gas andsaid fine particles to rise from said intermediate zone into theuppermost zone and discharging said fine particles laterally therefrom,and removing said gas trom the uppermost zone.

2. The method according to claim 1 characterized in that said zones arein a vertical column.

3. A method according to claim 1 characterized in that the verticaldistances between successive baffies in the zones above the lowermostzone increase in an upward direction.

4. A method according to claim 1 characterized in that the areas ofcommunication between successive zones are displaced angularly about 5.A method according to claim 1 characterized in that said zones arecylindrical in horizontal section and of uniform diameter.

6. A method according to claim 1 characterized in that the zones aredefined by horizontal disks having a sector removed therefrom.

7. A method according to claim 1 characterized in that the diameters ofsaid intermediate and upper zones are uniform.

8. The method according to claim 1, characterized in that said zones arein a vertical column, the vertical distances between successive bafliesin the zones above the lowermost zone increase in an upward direction,openings between successive zones are staggered causing the flow of saidmixture of gas and particles to follow a tortuous path upwardly andlaterally from side to side, the areas of communication betweensuccessive zones are displaced angularly about 90, said zones arecylindrical in horizontal section and of uniform diameter, the zones aredefined by horizontal disks having a sector removed therefrom, and thediameters of said intermediate and upper zones are uniform.

References Cited UNITED STATES PATENTS 2,483,485 10/1949 Barr 2092,621,118 12/1952 Cyr et a1. 34l0 X 2,666,526 1/1954 Odell et al 209138X 3,294,236 12/ 1966 Lagarias 209-l39 HARRY B. THORNTON, PrimaryExaminer.

TIM R. MILES, Examiner.

1. A METHOD OF FRACTIONATING MIXTURES OF PARTICULATE SOLIDS HAVINGCOARSE AND FINE PARTICLES, WHICH COMPRISES PROVIDING MEANS FORMING APLURALITY OF AT LEAST THREE SUPERPOSED ZONES IN COMMUNICATION WITH EACHOTHER AND SEPARATED BY A PLURALITY OF BAFFLES THEREBETWEEN DEFINING ANUPWARD TORTUOUS PATH FROM ONE SIDE OF A ZONE TO ANOTHER SIDE OF ANADJACENT ZONE, THE SPACE BETWEEN SAID BAFFLES INCREASING IN AN UPWARDDIRECTION AND SAID BAFFLES HAVING OPENINGS OF LIMITED SIZE ONLY AT THESIDES THEREOF, INTRODUCING A MIXTURE OF PARTICULATE SOLIDS INTO ONE OFSAID ZONES, INTRODUCING AN INERT GAS AT AMBIENT TEMPERATURES INTO THELOWERMOST ZONE AT A SUFFICIENT VELOCITY TO FLUIDIZE FINE PARTICLES OFSAID MIXTURE, SAI D VELOCITY BEING SUFFICIENTLY LOW TO CAUSE THECOARSEST PARTICLES FROM THE BOTTOM OF SAID LOWERMOST ZONE, CAUSING AMIXTURE CONTAINING SAID GAS AND SAID FINE PARTICLES TO RISE FROM SAIDINTERMEDIATE ZONE INTO THE UPPERMOST ZONE AND DISCHARGING SAID FINEPARTICLES LATERALLY THEREFROM, AND REMOVING SAID GAS FROM THE UPPERMOSTZONE.